Corticospinal excitability is not facilitated by observation of asymmetric walking on a split-belt treadmill in humans

The present study aimed to investigate changes in corticospinal excitability (CSE) by observing unnatural walking patterns on a treadmill with different left and right belt speeds. Fifteen healthy adults watched video clips (10 s each) of walking under the tied condition (left and right treadmill belt speeds are the same), walking during the initial and late periods under the split-belt condition (left and right treadmill belt speeds are different), and the static fixation cross (control condition) in random order. The step lengths of the actor in the walking clips were almost symmetric under the tied condition and during the late period under the split-belt condition but largely asymmetric during the initial period under the split-belt condition. We recorded the motor-evoked potential (MEP) of the left tibialis anterior muscle during the observation of video clips by delivering transcranial magnetic stimulation to the right primary motor cortex. The MEP amplitude was significantly higher when observing walking under the tied condition and walking during the late period under the split-belt condition than the control condition. However, the MEP amplitudes during the observation of walking during the initial period under the split-belt condition and the control condition were not significantly different. These results indicate that observation of symmetric walking, even under the split-belt condition, facilitates CSE, whereas observation of unnatural asymmetric walking does not. Therefore, it is suggested that familiarity with observed movements affects CSE even in the observation of semi-automatic movements such as walking.